1. Field of the Invention
The present invention relates to a diagnostic equipment for an exhaust gas cleaning apparatus installed in an engine system. More particularly, it relates to a diagnostic equipment for an engine exhaust gas cleaning apparatus which employs a catalyst together with a so-called "UEGO sensor(universal exhaust gas oxygen sensor)" that serves to measure an air fuel ratio in a wide range of air-fuel-ratio values or a so-called "O.sub.2 -sensor" (oxygen sensor) that generates a binary output on the basis of a sudden output change near a stoichiometric ratio (hereinbelow, both the sensors shall be collectively called an "air-fuel-ratio sensor").
2. Description of the Related Art
A well-known apparatus for cleaning the exhaust gas of an engine has a catalyst and an air-fuel-ratio feedback controller. The catalyst is incorporated in an exhaust pipe part for the purpose of eliminating HC (hydrocarbons), NO.sub.x (nitrogen oxides) and CO (carbon monoxide) which are contained in the exhaust gas. The air-fuel-ratio feedback controller is a device which is disposed for the purpose of causing the catalyst to demonstrate the function thereof satisfactorily, and which executes a control so as to hold the air fuel ratio of the exhaust gas at a predetermined value, while measuring the air fuel ratio by the use of an air-fuel-ratio sensor mounted upstream of the catalyst.
In such an exhaust gas cleaning apparatus, when the air-fuel-ratio sensor for measuring the air fuel ratio has undergone deterioration in its performance, the air fuel ratio sometimes deviates from the predetermined value, resulting in increase in the amounts of the noxious gas components contained in the exhaust gas. Moreover, the air fuel ratio may fall outside a ratio range in which the catalyst can demonstrate its performance, and the elimination efficiency (conversion efficiency) of the catalyst for the noxious gases may lower. On the other hand, when the catalyst itself has undergone deterioration in its performance, the conversion efficiency thereof for the noxious gases lowers in spite of the control of the air fuel ratio into the ratio range in which the catalyst can demonstrate its performance. In this manner, the deterioration of the performance of the air-fuel-ratio sensor or the catalyst results in increasing the amounts of the noxious gases which are emitted into the atmosphere. Therefore, a diagnostic equipment for the exhaust gas cleaning apparatus has been contrived in order to diagnose the performances during the drive of a vehicle furnished with the cleaning apparatus and to give warning to the driver of the vehicle against the deteriorations. By way of example, the diagnostic equipment is so constructed and operated that air-fuel-ratio sensors are respectively disposed upstream and downstream of the catalyst, and that an air-fuel-ratio feedback control is executed on the basis of, at least, the output of the air-fuel-ratio sensor located upstream of the catalyst, while the deterioration of the catalyst is detected on the basis of the output of the air-fuel-ratio sensor located downstream of the catalyst, etc. Such prior-art techniques are disclosed in the official gazettes of Japanese Patent Applications Laid-open No. 91440/1990 and No. 286160/1991.
With the method wherein, during the air-fuel-ratio feedback control, the deterioration of the catalyst is detected on the basis of the output of the air-fuel-ratio sensor located downstream of the catalyst, naturally the detection of the deterioration of the catalyst is impossible while the air-fuel-ratio feedback control is at rest. Besides, in such a case where the catalyst has not been activated yet, the detection of the catalyst deterioration is highly liable to err. It is therefore necessary to permit and inhibit the detection of the catalyst deterioration in accordance with the operating conditions of the engine, for example, the revolution speed (revolutions per minute) and load thereof. These factors are considered also in the prior-art technique disclosed in the official gazette of Japanese Patent Application Laid-open No. 91440/1990. Further, in a case where the air-fuel-ratio sensor located upstream of the catalyst has deteriorated, the detection of the catalyst deterioration is affected depending upon the extent or content thereof. Therefore, it is sometimes necessary to inhibit a correction for a detected result and the detection of the catalyst deterioration. In the prior-art technique disclosed in the official gazette of Japanese Patent Application Laid-open No. 286160/1991, accordingly, the detection of the catalyst deterioration is inhibited when the upstream air-fuel-ratio sensor has deteriorated.
Meanwhile, when the engine has misfired in the combustion stroke thereof, oxygen in air flows into an exhaust pipe together with unburnt gas. In consequence, the air-fuel-ratio sensor located upstream of the catalyst generates a spike signal indicating the lean exhaust gas, or the air-fuel-ratio sensor located downstream of the catalyst generates a signal indicating leaner exhaust gas than the actual one. This poses the problem that the accuracy of the detection of the catalyst deterioration lowers. Also in a case where a secondary air system disposed for introducing the air into the exhaust pipe has failed, the problem of the lowering of the detection accuracy is involved for such a reason that the air-fuel-ratio feedback control does not proceed normally. These drawbacks are not considered in any of the prior-art techniques.